1. Field of the Invention
The present invention relates to a gas turbine engine, and specifically to a process for determining an optimum rub of the blades in the compressor or turbine.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
In gas turbine engines, the gap between the rotating blades in both the turbine and the compressor is a critical design parameter. It is desirable for an efficiency standpoint to reduce the gap between the rotating blade and the adjacent static part in order to reduce leakage of the gas stream across this gap. The leakage not only reduces efficiency of the compressor or the turbine, but also reduces the life of the blade tips and shroud members because of high temperatures acting on the parts.
Some gas turbine engine designers will set the gap such that the blades will not rub at all. Some designers provide a negative gap in order to produce rub during the initial engine break-in in order to allow for the normal wear from the rub to produce a smooth and close to zero gap as possible. However, this rub is a complicated process, and several factors can effect how the rub should be performed to produce the best results. Certain materials used by the blade tips and the shroud members that will rub can have undesirable results if the rub produces too much heat. Galling can occur. Many factors must be evaluated in the operation of the gas turbine engine, such as thermal growths, rotational imbalances, centrifugal forces due to rotations, tolerances in the machining, gravitation effects on the casing, and even the uneven heating and cooling of parts of the engine. All these factors can have an effect on the rub produced under engine break-in procedures.
With conventional metals against metals, very high temperatures can be generated quickly and can cause severe damage to the metal components. When metal galling occurs, the damage can be even more extensive. Galling occurs when metals with dissimilar hardness (the blade tip and the shroud metal) come in contact at high speeds and pressures. Galling begins at metal defects such as notches or scratches and eventually develops into hard protrusions on the metal surface. These protrusions gouge the metal material. Factors that effect galling are relative speed between metals in contact, pressure between the metals, and lubrication between the metal contact surfaces.
It is therefore an object of the present invention to provide an improved process for optimizing a blade tip clearance in a gas turbine engine.